supernovae – the biggest bangs since the big bang kevin krisciunas, tamu

Supernovae – the biggest bangs since the Big Bang

Kevin Krisciunas, TAMU

A supernova is a star that ends its life in a fiery explosion.

There are different kinds of supernovae. Some are verymassive stars, as if we packed 20 stars like the Sun into asingle star. Other supernovae are members of close double star pairs, like the home stars of the planet Tatooinein the Star Wars movie series.

Our Sun will not end its life as a supernova. This weknow for certain.

The Earth is about 8000 miles in diameter, and the Sunis a little over 100 times the size of the Earth.

The center of the Sun is a nuclear fusion reactor. Forthe past 4.5 billion years it has been transforminghydrogen into helium by means of nuclear reactions.

Four hydrogen nuclei are fused into one helium nucleus.It turns out that one helium nucleus has 0.7 percentless mass than one four hydrogen nuclei. The mass isconverted into energy according to Einstein's famousformula E = mc2. For a little bit of mass, you get a lotof energy.

The Sun will continue converting hydrogen into heliumin its core until it is 10 billion years old.

When the Sun has used up the hydrogen in its core, itwill swell up to become a red giant star. It will becomea few dozen times its present size. This will result inthe end of life as we know it on the Earth. The oceanswill evaporate and the Earth will be much too hot tosustain life as we know it. But don't worry. That isroughly 6 billion years into the future.

At the end of the Sun's red giant phase, it will exhalemost of its atmosphere and produce what is known asa planetary nebula. This will leave a very small, hotstar in the middle – a white dwarf star. (In a smalltelescope a planetary nebula might look like a planet.The name is not quite right. We use it for historicalreasons.)

The “Southern CrabNebula” (left) andfour other planetarynebulae.

About 7 billion years from now the Sun will be a hot, densestellar remnant no bigger than the Earth. A white dwarf staris a million times denser than water! It is made up mostlyof carbon and oxygen.

Once the Sun becomes a white dwarf star, that will be itsend state. All it will be able to do then is slowly cooldown, forever.

But what if a star like the Sun had a close companion star?Something quite different could occur.

Imagine a star exactly like the Sun, with a close companionthat is slightly less massive. It turns out that the less massivestar uses up its nuclear fuel more slowly – it is a lower powernuclear reactor, but that phase of its life can last longer.

Eventually, the companionstar swells up too, and itcan pass matter (mostlyhydrogen) to the white dwarf star. This hydrogencould be converted intohelium on the surface ofthe white dwarf star. Thenwe'd observe a relativelysmall explosion.

However, if matter passes to the white dwarf star andit becomes more than 40 percent “heavier” than theSun, it will make a huge explosion. The entire whitedwarf will explode with the energy of four billionSuns. This is called a “white dwarf supernova”(also known as a “Type Ia supernova”).

Imagine you made a series of bombs, each with the sameamount of the same material. The bombs would allhave about the same power, right? The same is trueof white dwarf supernovae. If each one is the explosionof 1.4 solar masses of mostly carbon and oxygen, eachone gives us just about the same amount of energy.This means that they all have about the same intrinsicbrightness, like a 100 Watt light bulb, only much, muchbrighter.

Another way to blow up a star is to considerthe evolution of a star with 8 times the mass of theSun (or more). These stars are very rare. Only one ina thousand stars formed has this much mass.

An 8 solar mass star will not live for 10 billion yearsconverting hydrogen into helium in its core. It ishot enough in its core to convert hydrogen into helium,then helium into carbon, carbon into oxygen, oxygeninto neon, neon into magnesium, magnesium intosilicon, and eventually the core is made of iron.

Furthermore, all this takes place in 50 million yearsor less – a tiny fraction of the lifetime of the Sun.

Onion-like model of a massive star just before it explodes.

All the nuclear reactions in the core and shells of themassive star result in the conversion of some smallfraction of the material into energy according toEinstein's formula E = mc2. Once you end up withan iron core, any subsequent nuclear reactions woulduse up more energy than they produce.

The net result is that the light streaming from the coreof the massive star can no longer hold up the layersof the star above the core.

If the bottom layercannot hold up whatis above it, thewhole structurecollapses.

A massive star with an iron core lives for less than oneday. The outer layers squeeze down on the core andit explodes. Astronomers call this a “Type II supernova”.

Stars that end as Type II supernovae range in mass from8 to (maybe) 100 times that of the Sun. Because the amountof material that explodes varies from object to object,these supernovae do not produce an accurately predictableamount of light.

Another difference compared to “white dwarf supernovae”is that the single, massive stars that explode are notcompletely obliterated. They leave behind rapidly spinningneutron stars (pulsars), or black holes.

There have only been 8 bona fide supernovae observedin our Galaxy, the Milky Way. All of these supernovaewere seen before the invention of the telescope! Theywere observed in 185 AD, 386, 393, 1006, 1054, 1181,1572, and 1604. Most were only observed in China.

In the past century the galaxy NGC 6946 has hosted9 supernovae. They have been designated 1917A,1939C, 1948B, 1968D, 1969P, 1980K, 2002hh, 2004et,and the recent SN 2008S.

If some galaxies produce 9 supernovae per century,and it's been over 400 years since we had one in our Galaxy,don't you think we're overdue?

The two black tick marks indicate the location of SN 2008S.

Now, when you take an image of some galaxy witha telescope, in between you and that galaxy are alsoa lot of stars in our Galaxy.

To discover supernovae you need to image the sameplace on the sky again and again. Using a referenceimage, you can determine if a new star-like imageis an asteroid that just happens to be crossing thefield of view. Or maybe the new object is a variablestar in our Galaxy along the line of sight. Or maybeit is a supernova in the galaxy.

The galaxy M 100, asimaged in April of 2000(left). The numberedstars are in our MilkyWay galaxy in the samedirection as M 100.

The same galaxy in early2006. SN 2006X hasappeared near star 1.

The spectrum of SN 1999em obtained with the HubbleSpace Telescope and with a ground-based telescope.The bump at 6563 A is due to atomic hydrogen. Thiswas a Type II supernova.

Baron et al. (2000)

Spectra of twovery similar Type Ia supernovae.

The dip at 6150 Ais due to singlyionized silicon.This is the primesignature of aType Ia supernova.

Krisciunas et al. (2007)

When a white dwarf supernova explodes the primeproduct of the explosion is radioactive Nickel-56,which decays into Cobalt-56, then into Iron-56.(The number of protons plus the number of neutrons in the nuclei is 56.) Other, lighter atoms are distributed tointerstellar space too.

In a Type II supernova all the heavy elements up touranium are produced. All the gold and silver in yourjewelry or in the fillings in your teeth was produced inType II supernova explosions! We are literally “starstuff”. The single-star supernova also produce a hugenumber of particles called neutrinos. We detected suchparticles from the SN 1987A, which occurred in the Large Magellanic Cloud, a satellite galaxy of the Milky Way.

We have now described the two basic types supernovae.Some are explosions of white dwarf stars that have beenfed by companion stars until they exploded. Others aresingle, massive, stars that blow up on their own becausethey could not longer hold up all the layers of the star.

The identification of a supernova is accomplished byspreading out the light of the former star with aspectroscope, like spreading out a beam of sunlightwith a prism. We can also glean information aboutthe supernovae from plots of their brightness vs. time.Such measurements are made through certain speciallycolored filters.

This Type IIsupernovawas quiteconstant inbrightness for 3 monthsbefore it beganto fade


This white dwarfsupernova was observed in fiveoptical bands(UBVRI) and inthree infraredbands (JHK).The individuallight curves havebeen offset fordisplay purposes.Note the secondaryhump in the IJHKlight curves.


In 1929 Edwin Hubble discovered that the universe wasexpanding. Galaxies on average recede from us at aspeed proportional to distance. His first “Hubblediagram” was not particularly convincing, but we haveobserved much more distance objects since then.

These diagrams showthe infrared brightnessof white dwarf super-novae as a functionof velocity of recession.There is very littlescatter about the line.White dwarf supernovaeare very useful “standardcandles”.


If you know the apparent brightness of a star and youknow the intrinsic brightness of the star, you candetermine how far away the star is.

During the 1990's two groups of astronomers endeavoredto discover Type Ia supernovae as far away as possible.They found some objects so far away that their light hasbeen streaming towards us for 7 billion years. To observe such objects now is to look 7 billion years intothe past. (A telescope is a time machine, in effect.)

Under the assumption that the far away supernovae werejust like nearby ones of the same type, these two groupsof astronomers sought to confirm the rate of expansionof the universe and the extent to which the expansion wasslowing down.

All galaxies in the universe attract each other by means ofthe gravitational force. It made sense to think that thesum total of all this gravitational attraction would becausing the expansion of the universe to slow down.

However, when astronomers determined the distances tofaraway Type Ia supernovae, they found that on averagethey were too far away for their velocities of recession.Some mysterious force was causing an acceleration ofthe universe. We now refer to the cause of this accelerationas Dark Energy. It appears to be a property of the vacuumof empty space.

The universe is not slowing down. It is speeding up!

The attraction ofthe galaxies toeach other causedthe universe to beslowing down fora while, but theDark Energy eventually tookover. Now theuniverse isaccelerating.

Riess et al. (1998) and Perlmutter et al. (1999) providedevidence from Type Ia supernovae that the expansion ofthe universe was accelerating. Science rated this the “discovery of the year”.

We have just finished taking 6 seasons of observationswith a 4-m telescope situated at Cerro Tololo Inter-American Observatory in northern Chile. We discoveredover 200 Type Ia supernovae, some as far away as 7billion light-years.

Some of these objects have also been observed withthe Hubble Space Telescope and the Spitzer SpaceTelescope.


9 ESSENCE SN-ediscovered in2003 and observedwith HST.

Type Ib/c?

Redshift unknown

Why do so manyof these havesuch faint hosts?

Redshifts from0.53 to 0.79


Squares = ground-based data (largererror bars)

Triangles and dots =HST data

Was also observed with Spitzer SpaceTelescope.

Redshift 0.458 SNdiscovered in Dec.of 2005.

The evidence so far indicates that the universe willexpand forever. Many billions of years from now, whenall the gas and dust in the galaxies has been used up toform stars, and those stars have become white dwarfsor exploded as Type II supernovae, the universe willconsist of many faint galaxies that contain nothing butdim stars.

Right now, however, the universe is still processing itsraw material into stars. Some of those stars end theirlives in spectacular fashion. In one sense these explosionsbring death. In another sense they bring life. All theheavy atoms such as the calcium in your bones andthe iron in your blood were processed by stars billionsof years ago.